1. Field of the Invention
The present invention relates to the art of qualitatively measuring the properties of wood pulp in a dilute stock slurry. More specifically, the present invention relates to a method and apparatus for photometrically identifying the percentage of shives in a stock flow stream.
2. Description of the Prior Art
As a broad generalization, the process of pulping wood for papermaking comprises a series of chemical and mechanical steps to disintegrate the natural state of wood into individual or small bundles of cellulose fiber. However, since wood is not a homogenous material, the standardized or uniform process of an industralized pulp flow stream does not have the same result on all elements of a tree.
One particular notable natural wood anomaly is the occurrence of fiber bundles bound together by transverse ray cells. A singular, consolidated bundle grouping is known to the papermaker as a "shive." Such transversely bound bundles are extremely tenacious in their resistance to defiberizing processes.
Although wood pulp is normally screened one or more times along the process stream, it is impossible to segregate all shives having 4-8 length and 100 to 140 .mu.m width from acceptable fiber bundles of less than half the shive size. Nevertheless, it is important to the papermaker that the presence of shives be maintained below a certain percentage quantity in the stock due to their consequential deleterious effect on a paper web.
The generally practiced industry technique for monitoring the relative presence of shives in a pulp stream is to periodically count, manually, the individual shive incidence in a standard area, randomly selected, sample of paper made from the pulp. This practice, of course, is extremely time consuming and occasions a large delay interim between the time that pulp lands upon the papermachine and knowledgeable recognition of the relative shive incidence. Consequently, it is not unusual that machine operators will be plagued with web breaks unacceptable paper quality due to excess shives long before the cause of the trouble is known.
Recently, work has begun on the development of more timely techniques for shive monitoring. One published report of such work is found in the October, 1975 journal of the Technical Association Of The Pulp and Paper Industry (TAPPI, Volume 58, No. 10, page 120. This report describes an optical detector which passes two perpendicularly disposed light beams in a common plane transversely through a windowed conduit carrying a pulp sample stream. Respective photodetector responses to the shading effect of fibers crossing the beam paths are measured to yield a length, width and thickness determination for each fiber. By means of internally programmed limits, the event of a passing shive may be immediately identified and counted. The frequency of such shive counts is compared to the consistency and flow rate of the sample which must be carefully controlled.
Although the aforedescribed optical shive counter manufactured by Tellusond of Stockholm, Sweden, is extremely accurate, it is still a laboratory device which requires the isolation of a pulp sample from the production flow stream for accurate consistency and flow rate control.
Stock consistencies in a production flow stream are maintained in the range of 1 to 4% based on dry fiber weight. However, the Tellusond shive counter requires a batch quantity stock sample accurately measured to 0.01 g/l and a 10 minute processing period for each batch. These circumstances dictate an instrument preparation procedure which includes withdrawal of an adequate stock quantity of substantially unknown consistency from the production line, analyzing the sample for total fiber content and accurately mixing a sufficient stock quantity to an accurately known consistency. This procedure may be mechanized for an automatic sampling and measuring cycle but the necessary support apparatus is elaborate and subject to considerable maintenance.
U.S. Pat. No. 3,461,030 to M. A. Keyes describes a different type of wood pulp slurry measuring device which relies upon the dielectric quality of cellulose to impose a voltage variation between two electrodes as fibers suspended in a known electrolyte are passed therebetween. Similar to the Tellusond optical device, the Keyes instrument integrates a cross-sectional area measurement of an individual fiber with a transit time measurement to derive a volumetric conclusion. Consistency of the slurry is obtained by combining the fiber volume conclusion with a simultaneous slurry volume measurement. Although the Keyes instrument is disclosed in the context of a consistency measuring instrument, it is conceivable that it may be adapted to shive measurement simply because it obtains a quantitative measure of individual fiber or particle volume. Nevertheless, the inventor did not disclose a recognition of this capacity or how such an adaptation may be devised.
In light of such aforedescribed prior art, there heretofore remained a need for an instrument that will continuously measure the relative presence of shives in a mill production stock flow stream. The specification of this need by the pulp and papermaking industry is further complicated by the absence of a satisfactory pulp sampling technique that is simple, continuous and relatively maintenance free.
Cellulose fibers have an unusually high affinity for adhesion to each other and to foreign surfaces. Any surface exposed to a pulp stream is quickly coated with a layered growth of fiber. This growth continues until other forces such as gravity or fluid shear exceed the adhesion strength of the fiber bond thereby causing a breaking off of an accumulated quantity. Consequently, maintenance of a continuous flow stream of pulp is a scalar and velocity design problem. Relatively small pipelines will be quickly plugged by fiber accumulations if not self-cleaned by an appropriately high flow velocity. For these reasons it is difficult to continuously extract from a large production line a small but representative sample of pulp for testing purposes. Accordingly, it is also an objective of the present invention to teach a method and apparatus for continuously extracting a low quantity pulp sample that is representative of the primary flow stream but will not plug.